Ionic basis of the hyperpolarizing action of adenyl compounds on sinus venosus of the tortoise heart

Adenosine and the treatment of supraventricular tachycardia

Adenosine has recently become widely available for the treatment of paroxysmal supraventricular tachycardia. In order to evaluate its role in the management of arrhythmias, we have reviewed the literature on the cellular mechanisms, metabolism, potential for adverse effects, and clinical experience of the efficacy and safety of intravenous adenosine. Adenosine produces transient atrioventricular nodal block when injected as an intravenous bolus. This is of therapeutic value in the conversion to sinus rhythm of the majority of paroxysmal supraventricular tachycardias, which involve the atrioventricular node in a re-entrant circuit. The mean success rate was 93% from over 600 reported episodes. Compared with other antiarrhythmic agents, adenosine is remarkable for its rapid metabolism and brevity of action, with a half-life of a few seconds. It commonly produces subjective symptoms, particularly chest discomfort, dyspnea, and flushing, which are of short duration only. No serious adverse effect has been reported. Arrhythmias may recur within minutes in a minority of patients. Comparative studies have shown that adenosine is as effective as verapamil in the treatment of supraventricular tachycardia, and has less potential for adverse effects. Patients with supraventricular tachycardia should initially be treated using vagotonic physical maneuvers. Immediate electrical cardioversion is indicated if the arrhythmia is associated with hemodynamic collapse. Adenosine is the preferred drug in those patients in whom verapamil has failed or may cause adverse effects, such as those with heart failure or wide-complex tachycardia. The safety profile of adenosine suggests that it should be the drug of first choice for the treatment of supraventricular tachycardia, but only limited comparative data to support this view are available at present.

Three different potassium channels in human atrium. Contribution to the basal potassium conductance

We applied the cell-attached and inside-out patch-clamp technique under symmetrical isotonic potassium conditions on single human (and guinea pig) atrial cells. The human cells were isolated by a modified method to that described earlier. Our aim was twofold: 1) to study the single-channel characteristics of potassium channels in human atrial single cells, present under basal conditions (iK1 and iK(ATP] or when stimulated with 10(-5) M acetylcholine; and 2) to calculate the contribution of these three channel types to the total basal potassium conductance in human atrial cells, and to compare the results with data on guinea pig atrial cells under the same conditions. We found that in human cells 58% of the patches (n = 42/74) contained acetylcholine-sensitive potassium channels: their conductance was 42 +/- 1.2 pS and mean open time (tau o) was 1.7 +/- 0.5 msec. They showed sporadic openings in the absence of agonist, and activation by acetylcholine was G-protein dependent. In 16% of the patches (n = 7/44), adenosine (10(-4) M) activated the same channels, but the activity was lower than when stimulated by acetylcholine. In 18% of the patches (n = 9/51), an iK1 channel was present (conductance, 27 pS; tau o, 8.7 msec), whereas in the cell-attached mode, ATP-dependent channels were never seen. However, they were present in half of the inside-out patches on washout of ATPi (conductance, 73 pS; tau o, 1.4 msec). The basal potassium conductance (i.e., in the absence of any exogenous hormone or neurotransmitter) was mainly due to iK1 channels in both human and guinea pig cells, a finding that is in contrast with previous reports. However, the potassium current that is induced by acetylcholine is much higher in guinea pig than in human isolated cells; a fraction of it would suffice to fully determine the resting potassium conductance in guinea pig atrial cells, whereas it can play only a modulatory role in human cells. This difference could be important in species-specific autonomic modulation and antiarrhythmic drug action.

Adenosine or adenosine triphosphate for supraventricular tachycardias? Comparative double-blind randomized study in patients with spontaneous or inducible arrhythmias

The effects of intravenous adenosine and adenosine triphosphate (ATP) were studied in a double-blind randomized study during 68 episodes of supraventricular tachycardia in 39 patients. Adenosine restored sinus rhythm in 20 patients (25 of 27 episodes) and produced atrioventricular block to reveal atrial arrhythmias in nine. ATP restored sinus rhythm in 17 patients (22 of 25 episodes) and revealed atrial tachyarrhythmias in six. In patients receiving both compounds, the effective dosage of adenosine was 3.8 mg and of ATP it was 6.6 mg (p less than 0.05), suggesting molar equipotency. Transient side effects were common, occurring in 81% of episodes with adenosine and in 94% with ATP. Symptom scores (0 to 10) were not significantly different (median score for adenosine was 5, for ATP it was 6). Adenosine and ATP were equally effective for the diagnosis and treatment of supraventricular tachycardias and the incidence and severity of side effects were similar. Adenosine has the advantage of being more stable.

Membrane and intracellular effects of adenosine in mouse pancreatic beta-cells

Mouse islets were used to study the effects of adenosine and its stable analogue L-N6-phenylisopropyladenosine (L-PIA) on pancreatic beta-cell function. At a high concentration (500 microM), adenosine augmented glucose-induced electrical activity in beta-cells and potentiated insulin release. These effects were prevented by the inhibitor of nucleoside transport nitrobenzylthioguanosine. They probably result from the metabolism of adenosine by beta-cells. At a lower concentration (50 microM), adenosine caused a small and transient inhibition of glucose-induced electrical activity and insulin release. L-PIA (10 microM) slightly and transiently inhibited insulin release, 45Ca efflux and 86Rb efflux from islet cells, and decreased electrical activity in beta-cells. When adenylate cyclase was stimulated by forskolin in the presence of 15 mM glucose, insulin release was strongly augmented. Under these conditions, L-PIA and adenosine (with nitrobenzylthioguanosine) caused a sustained inhibition. No such inhibition was observed when insulin release was potentiated by dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP). These data are consistent with the existence of A1 purinergic receptors on mouse beta-cells. They could mainly serve to attenuate the amplification of insulin release brought about by agents acting via cAMP.

Value and limitations of adenosine in the diagnosis and treatment of narrow and broad complex tachycardias

The diagnostic and therapeutic potential of intravenous adenosine was studied in 64 patients during 92 episodes of regular sustained tachycardia. In 40 patients who had narrow complex tachycardias (QRS less than 0.12 s) adenosine (2.5-25 mg) restored sinus rhythm in 25 with junctional tachycardias (46 of 48 episodes) and produced atrioventricular block to reveal atrial or sinus tachycardia in 15. In 24 patients with broad complex tachycardias (QRS greater than or equal to 0.12 s) adenosine terminated the tachycardias in six patients and revealed atrial or sinus arrhythmias in four. The tachycardias persisted in 14 patients despite doses up to 20 mg, but adenosine allowed the diagnosis of ventricular tachycardia with retrograde atrial activation in two patients by producing transient ventriculoatrial dissociation. Diagnosis based on adenosine induced atrioventricular nodal block was correct in all patients with narrow complex tachycardias and in 92% of those with broad complex tachycardias, compared with correct electrocardiographic diagnoses in 90% and 75% respectively. Adenosine gave diagnostic information additional to the electrocardiogram in 25%. The response to adenosine in broad complex tachycardias identified those of supraventricular origin with 90% sensitivity, 93% specificity, and 92% predictive accuracy. Adenosine restored sinus rhythm in all patients with junctional reentrant tachycardias, but in 10 (35%) the arrhythmias recurred within two minutes. Symptomatic side effects (dyspnoea, chest pain, flushing, headache) were reported by 40 (63%) patients and, although transient, were severe in 23 (36%). There were ventricular pauses of over 2 s in 16% of patients, the longest pause being 6.1 s. Adenosine is of value in the diagnosis and treatment of narrow and broad complex tachycardias, but its use is limited by symptomatic side effects, a tenfold range in minimal effective dosage, occasional action at sites other than the atrioventricular node, and early recurrence or arrhythmia.

Characterization of adenosine receptors in guinea-pig isolated left atria

1. The effects of purinergic stimulation on action potential, force of contraction, 86Rb efflux and 45Ca uptake were investigated in guinea-pig left atria. 2. Adenosine exerted a negative inotropic effect which was antagonized by adenosine deaminase but enhanced by dipyridamole. 3. The negative inotropic effect of adenosine was mimicked by 5'-(N-ethyl)-carboxamido-adenosine (NECA) and the isomers of N6-(phenyl-isopropyl)-adenosine, R-PIA and S-PIA. NECA and R-PIA were about 100 times more potent than adenosine, whereas R-PIA was about 100 times more potent than S-PIA. 4. The inotropic effects of adenosine (in the presence of dipyridamole), NECA, R-PIA and S-PIA were competitively antagonized either by theophylline (pA2 about 4.5) or 8-phenyltheophylline (pA2 about 6.3). 5. NECA and R-PIA shortened the action potential duration and increased the rate constant of the efflux of 86Rb in a concentration-dependent manner with no differences in potency; the effects were competitively antagonized by 8-phenyltheophylline. 6. Barium ions reduced the efflux of 86Rb under control conditions and antagonized the increase induced by NECA and R-PIA. 7. NECA and R-PIA significantly reduced 45Ca uptake in beating preparations. 8. It is concluded that adenosine, NECA and R-PIA activate a common receptor population (P1 or A3) on the outside of the cell membrane of atrial heart muscle to increase the potassium conductance and to reduce the action potential and, thereby, calcium influx and force of contraction.

Comparison of the inhibition of insulin release by activation of adenosine and alpha 2-adrenergic receptors in rat beta-cells

Rat islets were used to compare the mechanisms whereby adenosine and adrenaline inhibit insulin release. Adenosine (1 microM-2.5 mM) and its analogue N6(-)-phenylisopropyladenosine (L-PIA) (1 nM-10 microM) caused a concentration-dependent but incomplete (45-60%) inhibition of glucose-stimulated release. L-PIA was more potent than D-PIA [the N6(+) analogue], but much less than adrenaline, which caused nearly complete inhibition (85% at 0.1 microM). 8-Phenyltheophylline prevented the inhibitory effect of L-PIA and 50 microM-adenosine, but not that of 500 microM-adenosine or of adrenaline. In contrast, yohimbine selectively prevented the inhibition by adrenaline. Adenosine and L-PIA thus appear to exert their effects by activating membrane A1 receptors, whereas adrenaline acts on alpha 2-adrenergic receptors. Adenosine, L-PIA and adrenaline slightly inhibited 45Ca2+ efflux, 86Rb+ efflux and 45Ca2+ influx in glucose-stimulated islets. The inhibition of insulin release by adenosine or L-PIA was totally prevented by dibutyryl cyclic AMP, but was only attenuated when adenylate cyclase was activated by forskolin or when protein kinase C was stimulated by a phorbol ester. Adrenaline, on the other hand, inhibited release under these conditions. It is concluded that inhibition of adenylate cyclase, rather than direct changes in membrane K+ and Ca2+ permeabilities, underlies the inhibition of insulin release induced by activation of A1-receptors. The more complete inhibition mediated by alpha 2-adrenergic receptors appears to result from a second mechanism not triggered by adenosine.

Somatostatin increases an inwardly rectifying potassium conductance in guinea-pig submucous plexus neurones

1. Intracellular recordings were made from neurones in the submucous plexus of the guinea-pig caecum and ileum. 2. Somatostatin hyperpolarized more than 90% of the neurones. The lowest effective concentration was 300 pM and the maximum hyperpolarization (about 30-35 mV) was caused by 30 nM. Under voltage clamp at -60 mV, somatostatin caused outward currents which reached a maximum of 350-700 pA. 3. The hyperpolarization or outward current reversed polarity at a membrane potential (about -90 mV in control solutions) which changed according to the logarithm of the external potassium concentration. 4. The somatostatin current showed inward rectification; when the inward rectification of the resting membrane was prevented by extracellular caesium or rubidium, the inward rectification of the somatostatin current also disappeared. 5. A potassium conductance with the same properties was increased by alpha 2-adrenoceptor agonists and by delta-opioid receptor agonists; however, the effects of somatostatin were unaffected by antagonists at alpha 2- or delta-receptors. The somatostatin analogue, cyclo-aminoheptanoyl-Phe-D-Trp-Lys-(benzyl)Thr, also did not antagonize the actions of somatostatin. 6. The hyperpolarization (or outward current) was unaffected by forskolin, cholera toxin, sodium fluoride, phorbol esters or intracellular application of adenosine 5'-O-(3-thiotriphosphate) (ATP-gamma-S). However, when the recording electrode contained guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) the hyperpolarizations reversed only partially when somatostatin application was discontinued, and repeated applications caused the membrane potential to approach and remain close to the potassium equilibrium potential. 7. It is concluded that somatostatin increases the conductance of a set of inwardly rectifying potassium channels in submucous plexus neurones. The coupling between somatostatin receptor and ion channel involves a guanosine 5'-triphosphate-binding protein, but is not likely to result from changes in intracellular levels of cyclic adenosine 3',5'-monophosphate.

Effect of adenosine on atrioventricular conduction. I: Site and characterization of adenosine action in the guinea pig atrioventricular node

Adenosine has a negative dromotropic effect and modulates hypoxia-induced atrioventricular (AV) conduction delay. To further characterize the negative dromotropic effect of adenosine in the guinea pig heart, we determined the site of adenosine-induced AV conduction block; the effect of uptake and deamination of adenosine on its concentration-negative dromotropic effect, and the adenosine receptor that mediates this action. In isolated AV node preparations (n = 16), adenosine in a dose-dependent manner decreased significantly the duration and amplitude of the action potential of atrionodal and nodal cells and, in addition, markedly depressed the maximum rate of rise of the action potential of nodal cells. At high concentrations (greater than 20 microM), adenosine rendered nodal cells inexcitable. In isolated perfused hearts (n = 7), adenosine (5.7 microM) prolonged total AV conduction time by 21 +/- 2 msec. Of this prolongation, 83% was due to an increase in the nodal-to-His-bundle interval and the remaining 17% to an increase in the atrionodal to nodal interval. Infusion of adenosine to cause a 50% increase (EC50) in atria-to-His bundle (AH) interval prolongation resulted in a perfusate (arterial) adenosine concentration of 5.0 +/- 0.6 microM and effluent (venous) adenosine concentrations of 2.8 +/- 0.4 microM, i.e., an arteriovenous difference of 44% (n = 4). When adenosine uptake and deamination were inhibited with dipyridamole (0.5 microM) plus erythro-9-(2-hydroxy-3-nonyl)adenine (5 microM), respectively, the EC50s were 0.28 +/- 0.02 (perfusate) and 0.32 +/- 0.03 microM (effluent). These data indicate that when nucleoside metabolism is inhibited, arterial and venous concentrations of adenosine reach equilibrium. In an additional 10 hearts, the following rank order of potency of adenosine agonists in causing AH interval prolongation was found: N6-cyclopentyladenosine greater than N6-(L-2-phenyl-isopropyl)adenosine greater than 5'-N-ethylcarboxyamidoadenosine greater than or equal to 2-chloroadenosine greater than adenosine, which is compatible with activation of an A1-type receptor. In summary: the site of adenosine-induced AV conduction block is the nodal zone of the AV node, when adenosine uptake and deamination are inhibited, adenosine in concentrations similar to that released by hypoxia causes significant AH interval prolongation, and the adenosine receptor mediating the negative dromotropic effect of adenosine is of the A1-type.